It is often beneficial for an application running on a computing device or a remote computing system that is interacting with a computing device to be able to identify the computing device. In the case of mobile computing devices such as cellular phones or other computing devices having cellular capability (e.g., cellular network enabled tablet devices), the mobile computing device can be identified by a phone number associated with the mobile computing device. One method for obtaining a phone number for a mobile computing device is to request that a user of the mobile computing device enter the phone number for the mobile computing device. For example, an application running on the mobile computing device can prompt the user to enter the phone number for the mobile computing device. The application can then cause the mobile computing device to transmit the entered phone number to a remote computing system, such as an application server associated with the application. The remote computing system can use the phone number to identify the mobile computing device.